1. Field of the Invention
The present invention is directed to a gradient coil system, particularly for a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technology for acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus further has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and that picks up the generated magnetic resonance signals, from which magnetic resonance images are generated.
The gradient system includes gradient coils and controlled gradient amplifiers. Each of the gradient coils generates a gradient field for a specific spatial direction, this gradient fieldxe2x80x94in the desirable ideal casexe2x80x94being formed exclusively comprises a main field component that is co-linear with the basic magnetic field at least within the imaging volume. The main field component has a prescribable main gradient that is of approximately the same magnitude, independent of location, at any arbitrary point in time, at least within the imaging volume. Since the gradient field is temporally variable magnetic field, the above characteristics are applicable in fact applies for every point in time; however, the strength of the main gradient is variable from one point in time to another point in time. The direction of the main gradient is usually rigidly prescribed by the design of the gradient coil.
Due to Maxwell""s fundamental equations, however, and contrary to the desired, ideal case, no gradient coil can be fashioned that exclusively generates the aforementioned main field component over the imaging volume. The main field component is unavoidably accompanied by at least one secondary field component that is perpendicular to the main field component.
For generating the gradient field, suitable currents are set in the gradient coil. The amplitudes of the required currents amount to several 100 A. The current rise and decay rates (slew rate) amount to several 100 kA/s. The gradient coil is connected to a controlled gradient amplifier for the power supply.
In magnetic resonance image exposures, stimulations can be triggered in a living examination subject by the switching of the gradient fields. The gradient fields acting on the examination subject are characterized by a time-varying, magnetic flux density that generates eddy and induction currents in the examination subject. The strength of these electrical currents is dependent on, among other things, the cross-sectional area that the gradient field penetrates as well as on the time variation of the gradient field. These currents flow through regions of the examination subject with different electrical conductivity and thereby produce corresponding electrical voltages. When the voltage exceeds a specific threshold, this leads to the triggering of stimulations of the examination subject. For example, German OS 42 25 592 discloses an arrangement wherein the highest current or voltage values given switched gradient fields are induced at the edge of or outside the imaging volume where the field boost of the magnetic flux density of the gradient field is maximum, so that the risk of stimulations is greatest there.
In order to avoid such stimulations, German OS 42 25 592 discloses that stimulation-sensitive regions outside the imaging volume be covered with a closed conductor loop. A reduction of the currents induced in the covered region results therefrom. These coverings, however, are only possible outside the imaging volume and also are not possible in edge regions of the imaging volume because the linearity of the gradient fields in the imaging volume, which is important for the image quality, and the homogeneity of the basic magnetic field are otherwise deteriorated. It is also disadvantageous that the position of the conductor loops usually also must be adapted given a change of the region of the examination subject to be imaged.
In the context of a hollow-cylindrical gradient coil system, German OS 195 27 020 discloses a combination of a segment gradient coil and a gradient coil constructed of saddle-shaped sub-coils for a transverse gradient coil. The advantages of both types of gradient are intended to be preserved and their disadvantages reduced at the same time due to the combination. One disadvantage of the gradient coil constructed of saddle-shaped sub-coils is stated to be that it exhibits a strong accompanying field component that is stronger than the usable main field component. Among other things, the accompanying field component and, thus, the risk of stimulations due to rapidly switched gradients are intended to be noticeably reduced by the combination.
Further, a shielding arrangement is known, for example from European Application 0 317 853 or British Specification 2 207 764 which is intended to prevent a field emanating from a current-permeated gradient coil system from penetrating into parts of a basic field magnet system that surrounds the gradient coil system.
An object of the present invention is to provide an improved gradient coil system with which, among other things, high intensities of a rapidly switched gradient field can be achieved without causing stimulations in a living examination subject.
This object is achieved in a gradient coil system according to the invention having at least one gradient coil for generating a magnetic gradient field at least within an imaging volume, the gradient field having a main field component co-linear with a basic magnetic field and at least one accompanying field component perpendicular to the main field component, and an electrically conductive shield for shielding the accompanying field component, at least toward the imaging volume.
The inherently undesired accompanying component thus can be attenuated at least in a region in which a living subject is to be examined with the magnetic resonance technique. As a result, the gradient field permeating the examination subject can be reduced to the main field component effective for magnetic resonance imaging and thus a stimulation probability of the examination subject can be diminished, or the usable intensity of the gradient field can be increased. One advantage of the shield is that its effect is limited only to fast changes of the gradient field that are particularly responsible for stimulations. The maximally obtainable gradient strength of a gradient filed for a longer time segment, wherein the strength does not vary over time, is not reduced.